The present invention relates to a new process for the manufacture of mercaptocarboxylic acids represented by the formula (I): 
in which:
R1 represents hydrogen or 
xe2x80x83and
R2 represents hydrogen or methyl.
These mercaptocarboxylic acids are useful as intermediates for synthesis into the esters (for example with 2-ethylhexanol, pentaerythritol) used either to obtain tin salts (stabilizers for PVC), or as hardening or modifying agents for epoxide resins (adhesives) and for polyurethanes (optical glasses). Mercaptopropionic acid can also serve to modify acrylic polymers (paint, paper industry).
Various routes are known for producing mercaptocarboxylic acids:
(1) One of these routes consists in reacting halogenated saturated carboxylic acids with alkali metal hydrosulphides and under an H2S pressure. This route is widely described in the literature for thioglycolic acid from monochloroacetic acid, and for mercaptopropionic acid from 3-chloropropionic acid (see American patent U.S. Pat. No. 3,927,085). It is known that this reaction is very selective if the operating conditions are well chosen. However, the stoichiometry of the reaction requires the use of 2 mol of alkali metal hydrosulphide per mol of chlorinated carboxylic acid and the consequent formation of 2 mol of inorganic salt to be removed in the aqueous effluent.
(2) Reactions for opening the lactone rings by alkali metal hydrosulphides have also been proposed; thus, the formation of xcex2-mercaptopropionic acid from xcex2-propiolactone and NaSH is described in British Patent No. 639 679. It should be indicated here that propiolactone is not commercially available.
(3) The preparation of 3-mercaptopropionic acid can also be carried out from widely available raw materials such as unsaturated organic acids and, in particular, the acrylic derivatives.
The reaction of H2S in a basic medium or of alkali metal hydrosulphides makes it possible to obtain as an intermediate, starting with acrylonitrile, mercaptopropionitrile which can be subsequently hydrolyzed to 3-mercaptopropionic acid.
However, very special conditions should be observed in order to avoid the very easy reaction of the mercaptopropionitrile formed with the unreacted acrylonitrile. The thiodipropionitrile sulphide is indeed very easy to form and it is then necessary to subsequently carry out a sulphohydrolysis of the sulphide to mercaptan, thereby complicating the preparation process.
Moreover, in addition to going via toxic and unstable products (mercaptopropionitrile), this process has the disadvantage that 2 mol of salt will be further formed per mol of 3-mercaptopropionic acid.
Another possibility is that consisting of using, as raw material, methyl acrylate, and to form, by reaction with H2S, methyl mercaptopropionate, which, by hydrolysis, will lead to 3-mercaptopropionic acid. This route is economically unattractive because it occurs in two stages, with the loss of one mol of methanol.
A direct route for preparing mercaptopropionic acid consists in starting with acrylic acid, which is a cheap raw material. The reaction of acrylic acid in liquid phase, with a large excess of H2S in the presence of an organic base as catalyst, is not easy to carry out. The other technique, consisting in reacting acrylic acid with CS3Na2, makes it possible to enhance the selectivity in relation to mercaptopropionic acid and to carry out the procedure without an H2S pressure. Unfortunately, the use of CS2 and of methanol as cosolvent makes the process cumbersome, especially with regard to the treatment of effluents.
GB-A-670 702 describes, in its example I, the following reaction: 
In fact, thiodisuccinic acid 
which undergoes sulphohydrolysis to mercaptan because of the basic pH, is first prepared. In other words, generally speaking a sulphide is first formed, which sulphide undergoes sulphohydrolysis to mercaptan according to the reaction scheme: 
A, R1, R2 being as defined below.
Applicant has, moreover, experimentally confirmed this reaction scheme. The article in Collection of Czechoslovak Chemical Communications, Vol 21, No. 6, 1956, Prague CS, pages 1651-1653 presents what is described in GB-A-670 702.
GB-A-1 358 019 describes a process for the preparation of xcex2-mercaptopropionic acid by reaction of acrylic acid with MSH where M=ammonium group or alkali or alkaline-earth metal in the presence of carbon disulphide CS2. CS2 plays the role of coreagent and prevents the sulphide from being formed: 
In accordance with the documents which have just been cited, the aim is to obtain a better selectivity in relation to mercaptocarboxylic acids and to avoid the formation of sulphides. In fact, the aim would be to avoid the formation of sulphides by reaction of the mercaptan already formed and the nonconverted acrylic compound: 
with 
or HOOCxe2x80x94and R2=H or CH3.
The present invention is essentially centred on the beneficial effect of the solubility of H2S in the reaction medium which promotes the formation of mercaptan. In accordance with the present invention, carrying out the procedure with a supply of H2S leads directly to the desired mercaptan, the sulphohydrolysis of the sulphide 
becoming impossible under these conditions.
According to the invention, hydrosulphides are therefore reacted with unsaturated carboxylic acids, according to the following overall reaction (case where the hydrosulphide is represented by ASH): 
A=ammonium or alkali metal cation;
R1=H or 
xe2x80x83and
R2=H or CH3 
which is, in fact, the sum of the following two reactions: 
It was surprising to observe that, under the abovementioned conditions, the mercaptan formed reacts weakly with the unsaturation of the acid R1xe2x80x94CHxe2x95x90CR2xe2x80x94COOH, whereas the reaction of formation of the mercaptan is itself highly favoured. This results in very good selectivities in relation to the desired compound of formula (I) which are higher than those of known processes.
The subject of the present invention is therefore a process for the manufacture of a mercaptocarboxylic acid represented by the formula (I) as defined above, according to which an unsaturated carboxylic acid of formula (II): 
in which R1 and R2 are as defined above, is reacted with a hydrosulphide of formula ASH, A being an alkali metal cation or a cation NR3R4R5R6+, R3 to R6 each representing H or a hydrocarbon radical, or of formula Q(SH)2, Q being an alkaline-earth metal cation, and the resulting reaction medium is acidified in order to obtain the desired compound (I), characterized in that the reaction is carried out with a supply of H2S other than that supplied by the neutralization of the acid (II).
The starting acid (II) is chosen especially from acrylic acid, methacrylic acid, maleic acid and fumaric acid.
In general, the reaction according to the invention may be carried out in an aqueous medium, but an enhancement of the selectivity can be obtained using an aqueous-alcoholic or a purely alcoholic medium. This alcoholic medium, or the alcohol fraction of the aqueous-alcoholic medium, is formed, for example, by methanol, propanol, isopropanol or a mixture of these alcohols.
An important parameter which governs the selectivity in relation to mercaptocarboxylic acid (I) is the H2S/acid (II) molar ratio in solution. The higher the H2S concentration in solution, the better the selectivity in relation to the acid (I). If the procedure is carried out without a supply of H2S other than that supplied by the neutralization of the acid (II), this ratio is 1. If H2S is supplied to the reaction system, its solubility in the reaction medium depends on several physical factors:
dilution of the medium (true H2S/acid (II) molar ratio in solution will be higher);
use of an alcoholic or aqueous-alcoholic medium;
increase in the H2S pressure; and
to a lesser degree, reduction in the temperature; as well as on the H2S/acid (II) ratio in solution.
To increase this ratio, as is envisaged by the present invention, H2S may be introduced into the medium by a direct external supply and/or H2S may be produced in situ by reacting the hydrosulphide with at least one acid introduced into the medium, in accordance with the following reaction (with ASH): 
This acid may be chosen from the saturated organic acids of formula RCOOH, R representing especially a C1-C18 alkyl residue capable of carrying substituents such as halogens, and from inorganic acids.
The acid RCOOH is advantageously chosen from acetic acid, propionic acid and 3-chloropropionic acid. In the case where it is desired to obtain 3-mercaptopropionic acid, the procedure is advantageously carried out in the presence of 3-chloropropionic acid which will itself lead to the desired 3-mercaptopropionic acid, in accordance with the following reactions:
2ASH+Clxe2x80x94CH2CH2xe2x80x94COOHxe2x86x92HSxe2x80x94CH2xe2x80x94CH2xe2x80x94COOA+H2S +ACl
ASH+CH2xe2x95x90CHxe2x80x94COOHxe2x86x92HSxe2x80x94CH2xe2x80x94CH2xe2x80x94COOA
At least one inorganic acid, such as HCl, H2SO4, and the like may also be used in place of the organic acid.
The pH should advantageously be from 6.5 to 8, and preferably from 6.8 to 7.5 in the reaction medium under an H2S pressure so as to avoid nucleophilic attack of the xe2x88x92Sxe2x80x94Yxe2x80x94CO2xe2x88x92 species (Y=CHR1xe2x80x94CHR2) on the double bond to give the sulphide.
In accordance with another characteristic of the process according to the invention, the procedure is advantageously carried out under an H2S pressure of at least 8 bar, especially from 8 to 30 bar. As regards the reaction temperature, it is advantageously between 10 and 150xc2x0 C. In the case of the preparation of 3-mercaptopropionic acid, this temperature is generally from 20 to 100xc2x0 C.
The acid (II) concentration, expressed in moles per liter of solvent (water or alcohol or water+alcohol) is generally between 0.5 and 4.
The hydrosulphide ASH or Q(SH)2 may be prepared separately and injected into the reactor, or may be formed in situ, at least in part, by reaction in an aqueous or alcoholic or aqueous-alcoholic medium of H2S with AOH or Q(OH)2, A and Q being as defined above. Thus, the hydrosulphide may be obtained by reaction of H2S with sodium hydroxide, potassium hydroxide, lime, amines and ammonium hydroxide.
One of the advantages of the process in accordance with the present invention is that it can be applied in existing industrial plants already optimized for the synthesis of mercaptocarboxylic acids from chlorinated carboxylic acids (thioglycolic acid for example).
It should also be emphasized that the process of the invention is carried out in an aqueous or aqueous-alcoholic or alcoholic medium without using CS2 and without the process being carried out in a large excess of H2S. Another economic advantage is that only one molecule of salt per molecule of mercaptocarboxylic acid is formed if H2S from an external source is used, in contrast to processes starting with halogenated carboxylic acids which, during their conversion to mercaptans, form 2 mol of salt which has to be removed.